We develop a paraxial theory of spin dynamics for twisted electrons traversing the entrance or exit fringe of an axially symmetric magnetic coil. Starting from the Foldy-Wouthuysen reduction, we identify a transverse Pauli coupling proportional to the fringe-field gradient. The scalar orbital dynamics is treated exactly through the Ermakov mapping, while the transverse spin coupling is included perturbatively, yielding explicit first-order transition amplitudes for general solenoidal profiles. Axial symmetry requires each spin flip to be accompanied by a compensating change of orbital angular momentum, while the coordinate-linear perturbation restricts each incoming oscillator mode to at most two neighboring radial sidebands. For 100-keV electrons in a 1-T field, the low-order examples considered give integrated spin-flip probabilities of order ${10}^{-9}$ per edge. The framework therefore quantifies mixing between spin and orbital angular momentum (OAM) in realistic magnetic lenses and provides a basis for controlling spin-OAM conversion using engineered sequences of magnetic-field edges.

The one-center Coulomb–Sturmian convergent close-coupling method is applied to proton collisions with the boron atom and singly charged carbon ion. Here we report an update to our target-structure implementation, in which configuration state functions are constructed using the method of coefficients of fractional parentage. To assess the quality of the structure models for the two targets, we present the excitation energies, oscillator strengths, and dipole polarizabilities obtained from the present configuration interaction calculations. Cross sections for total and state-selective target excitation and electron loss are calculated from 10 keV to 1 MeV. For both systems, the total excitation cross section is found to be dominated by excitation of the $2s$ subshell. This emphasizes the importance of a multi-electron description of the target in such scattering calculations. Comparisons with previous theoretical and experimental data are presented and discussed. In particular, we find that the present calculation for the electron-loss cross section in $p+{\text{C}}^{+}$ collisions is in good agreement with the available measurements across the entire overlapping incident-energy range.

Recent real-time, time-dependent density functional theory (TDDFT) calculations have reported an unexpected delayed growth of molecular dipole oscillations some time after an extreme-ultraviolet (XUV) pulse is applied. We show that numerical and analytical arguments suggest that this instability is an artifact of an incorrect nonlinearity introduced by the computational approach: Propagation with an adiabatic exchange-correlation approximation within the time-dependent Kohn-Sham equations of TDDFT tends to amplify initially small and pure sinusoidal oscillations in a system. On the other hand, when this same adiabatic approximation is used within the recent response-reformulated TDDFT, the instability is absent. The absorbing boundary condition plays a crucial role consistent with our argument. We demonstrate this explicitly on the ${\mathrm{N}}_2$ molecule subject to an XUV pulse, with a range of adiabatic functionals.

Double and triple photoionization of Be-like ${\mathrm{B}}^{+}$ ions have been studied in the photon-energy range from approximately 250 to 1300 eV. The method of merged photon and ion beams was employed and calculations were performed in support of the experiments making use of different theoretical approaches. The cross section ${\sigma }_{13}$ for net double ionization in the investigated photon-energy range is dominated by direct removal of one $K$-shell electron and subsequent single-Auger decay. $K$-shell ionization is described very well by theory. Accordingly, measured yields of ${\mathrm{B}}^{3+}$ photoproducts were normalized to theory to obtain absolute cross sections ${\sigma }_{13}$. Aside from the process of direct single ionization plus autoionization, ${\sigma }_{13}$ additionally contains contributions from photoexcited double-$K$-shell-vacancy ${\mathrm{B}}^{+}$ resonances, from direct double $L$-shell ionization, and from direct double ionization involving one $L$-shell and one $K$-shell electron. We show that, apart from single $K$-shell ionization with subsequent Auger decay, all these processes are of minor importance for ${\sigma }_{13}$. Nevertheless, the sensitivity of our experiment allowed us to observe the $2s^{2}2p3s{}^{1}P_1$ double-$K$-shell-vacancy resonance and to determine its energy, width, strength, and asymmetry parameter. Relativistic Hartree-Fock theory was employed to characterize the role of such resonances in ${\sigma }_{13}$. The cross section ${\sigma }_{14}$ for net triple ionization of ${\mathrm{B}}^{+}$ is expected to arise primarily from direct double $K$-shell ionization and subsequent single-Auger decay. The experimental cross section is found to be larger than theoretical predictions. Possible additional contributions to ${\sigma }_{14}$ such as $K$-shell ionization with accompanying $K$-shell excitation and subsequent Auger decay are discussed, as is the influence of metastable ions in the primary ${\mathrm{B}}^{+}$ ion beam.

We demonstrate the time-resolved detection of dynamical phase transitions (DPTs) in lattice-confined spinor gases subject to a priori unknown time-variant interactions, via the temporal behaviors of both the system energy and spinor phases extracted from the observed spin dynamics. Using this technique, we describe the observed nonequilibrium spin dynamics, governed by intricate spin-spatial couplings, across a range of conditions. This work also introduces a diagnostic tool that can quickly identify DPTs without requiring observations over at least a full period of the spin dynamics. Our approach can naturally extend to other complex systems subject to time-dependent parameters, such as Floquet systems under driven magnetic fields, driven interactions, or spin-flopping fields, with potential applications in the study of DPTs in nonintegrable models.

The superradiant phase transition in the dissipative Dicke lattice model, driven by on-site collective atom-photon couplings and intersite photon hopping, is a cornerstone of nonequilibrium quantum many-body physics. However, little is still known about the influence of boundaries in experimental achievable systems of finite size. Here we investigate the dissipative superradiant phase transition in the Dicke lattice model with a small number of sites and reveal a striking sensitivity of this model to the nature of the boundary conditions. Under open boundary conditions, we observe qualitatively distinct behavior, including the absence of homogeneous superradiance, a strongly shifted critical coupling, and an enlarged range of hopping strengths over which steady states remain stable, in sharp contrast to that with periodic boundary conditions. Our results demonstrate the crucial influence of boundary effects on the stationary phases of dissipative lattice models, which offers intriguing opportunities for studying these phenomena in near-term experimental realizations of such models in quantum optics and circuit QED.

We present a theoretical study of the two-particle spectrum $\omega \left(K\right)$ for the chiral waveguide QED setup of an array of two-level atoms directionally interacting with photons propagating along the waveguide. We demonstrate that for each pair center-of-mass momentum $K$ there exist distinct solutions with $\mathrm{Im}\omega \le 0$ in the two-particle spectrum, corresponding to bound, antibound, and resonance states, in addition to the continuum of scattering states. Contrary to previous studies, which showed the bound and resonance-state spectra only over a limited range of $K$, the calculated spectrum is consistent across all $K$ values. An interesting finding is that the real part of the spectrum $\text{Re}\omega \left(K\right)$ in the chiral model is gapless. The calculated dispersion law $\omega \left(K\right)$ provides an effective model for the bound photon pairs also in a finite-size array, manifesting the topological non-Hermitian skin effect.

Many-body quantum systems with local interactions undergo “sudden death of entanglement” at high temperatures, whereby thermal states become classical mixtures of product states. We investigate whether symmetry constraints can prevent this phenomenon. We prove that strongly symmetric thermal states (canonical ensemble) of generic Hamiltonians with on-site Abelian symmetries remain entangled with nonzero entanglement negativity at arbitrarily high temperatures, under mild conditions on the symmetry actions and the charge sector of the strong symmetry. Our results extend to weakly symmetric thermal states (Gibbs ensemble) under superselection rules, which restrict state decompositions to be symmetric. In particular, we show that fermionic Gibbs states evade sudden death of entanglement and have persistent fermionic negativity at high temperatures, proving along the way some existing conjectures about fermionic entanglement. These findings demonstrate that global symmetry correlations can preserve quantum entanglement despite thermal decoherence, providing new insights into the interplay between symmetry and quantum information in thermal equilibrium.

In this work we investigate the effect of decomposition basis on primitive qudit gates on superconducting radio-frequency cavity-based quantum computers with applications to lattice gauge theory. Three approaches are tested: snap and displacement gates, ecd and single-qubit rotations $R(\theta ,\varphi )$, and optimal pulse control. For all three decompositions, implementing the necessary sequence of rotations concurrently rather than sequentially can reduce the primitive gate run time. The number of blocks required for the faster ecd and $R_p\left(\theta \right)$ is found to scale as $O\left(d^2\right)$, while the slower snap and displacement set scales sublinearly with $d$ on the permutation gates relevant for lattice gauge theory. For qudits with $d<10$, the resulting gate times for the two decompositions are similar, though strongly dependent on experimental design choices. Optimal control can outperform both decompositions for small $d$ by a factor of 2 to 12 at the cost of higher classical resources. Lastly, we find that snap and displacement are slightly more robust to a simplified noise model.

Quantum error correction is essential for fault-tolerant quantum computation, yet most existing codes rely on local control and stabilizer measurements that are difficult to implement in systems dominated by collective interactions. Inspired by S. Omanakuttan and T. J. Volkoff, Spin-squeezed Gottesman-Kitaev-Preskill codes for quantum error correction in atomic ensembles, Phys. Rev. A 108, 022428 (2023), we develop a general framework for Holstein-Primakoff (HP) spin codes that maps continuous-variable bosonic codes onto permutation-invariant spin ensembles via the Holstein-Primakoff approximation. We show that HP codes are robust to both collective and local spin noise and propose an explicit measurement-free local error recovery procedure to convert local noise into correctable collective spin errors.

We develop a general perturbation theory for the local quantum uncertainty (LQU), a discord-type quantifier of nonclassicality based on the Wigner-Yanase skew information. Starting from a perturbed density matrix $\rho ={\rho }_0+\epsilon {\rho }_1$, we derive an explicit first-order expansion of ${\rho }^{1/2}$ using an integral representation based on the gamma function, and reduce the LQU optimization to the diagonalization of a $(d_1^2-1)\times (d_1^2-1)$ matrix $w=w^0+w^1$ defined in terms of the generators of $\mathrm{SU}\left(d_1\right)$. The resulting framework is valid for composite systems of arbitrary dimension $d_1\times d_2$ and provides a direct computational route to the LQU from the spectral decomposition of the unperturbed state ${\rho }_0$. We further specialize the theory to the quantum linear-response regime, where the perturbation ${\rho }_1$ is generated by a time-dependent external field coupled to the system through an operator $\widehat{A}$. In this regime, $w^1$ acquires an explicit dependence on the driving frequency $\omega$, the eigenstates and occupation probabilities of the equilibrium Hamiltonian $H_0$, and the matrix elements of $\widehat{A}$, establishing a direct link between the LQU and the spectral and dynamical structure of the system. The formalism is applicable to a broad class of physical models, including quantum spin arrays with anisotropic and spin-orbit interactions, atoms coupled to cavity fields, and optomechanical systems. To illustrate the approach, we apply the theory to the isotropic Heisenberg model of two coupled spins subject to a local periodic magnetic field, deriving closed-form analytical expressions for the LQU as a function of temperature $T$ and frequency $\omega$. Comparison with the concurrence shows that, above the entanglement critical temperature $T_c$, the external perturbation induces a resonantly enhanced quantum discord without generating entanglement. This demonstrates that frequency acts as a tunable modulator of nonclassicality in the linear-response regime, an effect that is purely of quantum-discord type and a property inaccessible to entanglement-based quantifiers.

We uncover a mechanism for realizing three-component quantum droplets in ultracold Bose gases, where only one interspecies interaction is attractive. In this scheme, the interspecies attraction leads to a self-bound binary droplet, and the third component is incorporated through Rabi coupling with one component of the binary droplet. We find that a stronger Rabi coupling leads to a larger fraction of the third component, but also destabilizes the entire droplet due to the involvement of more repulsive forces. Such instability can be remedied by a finite detuning between the Rabi-coupled components. We demonstrate these results in realistic Na-Rb mixtures, using both thermodynamic analyses and numerical simulations based on extended Gross-Pitaevskii equations. Our work outlines a general route for stabilizing multicomponent droplets by bridging an existing binary droplet with additional components via suitable single-particle fields.

We investigate the pairing problem in a two-component Fermi gas subject to periodically modulated attractive interactions. For the two-body case, we construct the Floquet bound state and find that its quasienergy decreases with the (dimensionless) driving strength, a manifestation of the renormalized $s$-wave scattering length in the high-frequency limit. For the many-body case, we find that, in sharp contrast to the static limit, the order parameter acquires nonzero time-periodic components that significantly modify the quasiparticle excitation spectrum with avoided crossings, where characteristic Floquet-induced gaps open at the boundaries of the Floquet Brillouin zone. The unique features of these unusual Floquet two-body and many-body states are further revealed by the spectral function and condensate fraction, which can be measured in experiments. Our results pave a different way for exploring nonequilibrium Fermi superfluid beyond static system through Floquet engineering.

We investigate shortcut-to-adiabaticity protocols for the rapid compression of optical solitons in nonlocal media. Using a variational approximation, we derive an effective Ermakov-like equation for the soliton width and employ inverse engineering to design propagation-dependent control parameters. Three control strategies are compared, based respectively on the modulation of the characteristic nonlocal length, the Kerr nonlinearity, and the external parabolic confinement. We show that all three protocols enable high-fidelity compression over strongly reduced propagation distances. By comparing control smoothness and final fidelity, we find that the relative performance of the protocols in the very short-distance regime depends on the chosen criterion, revealing a trade-off between implementation smoothness and target-profile accuracy. These results establish a practical and general route to fast soliton manipulation in nonlocal media.

We investigate the phase-dependent optical response and sensing capabilities of a V-type quantum emitter positioned near a graphene monolayer. The graphene sheet creates an anisotropic electromagnetic environment that modifies the spontaneous emission rates and induces correlated decay between the two optical transitions of the emitter. For the graphene parameters considered here, the resulting interference parameter is $p\simeq 0.33$. We show that this graphene-induced interference makes the component-resolved optical response highly sensitive to the relative phase and amplitude ratio of two circularly polarized driving fields. Using a density matrix model that includes pure dephasing, we derive analytical conditions for phase-controlled zero absorption with finite dispersion, complete transparency, and negative effective absorption of one field component. The latter corresponds to phase-sensitive amplification without population inversion, with the second coherent component supplying the transferred energy, while the total two-field absorption remains non-negative. We then show how the phase-selected spectral features can be used as frequency-readout observables for sensing. Zeeman splitting of the upper states shifts the transparency point and provides a route to optical magnetometry, while changes in the local dielectric environment modify the graphene-induced decay constants and enable refractive index sensing. The analysis identifies the dephasing-limited threshold for exact transparency and discusses linewidth and resolution constraints. The proposed system provides an electrically tunable graphene-based platform for coherent optical control and nanoscale quantum sensing.

We analyze propagation and detection of two-photon states expanded in Zernike modes through atmospheric turbulence using the extended Huygens-Fresnel formalism. For spontaneous parametric down-conversion states prepared with a single Zernike pump mode, we analytically reduce the eight-dimensional continuous propagation integrals to an exact, discrete modal expansion. In the absence of turbulence, Zernike addition enforces conservation of azimuthal index and a strict radial-order bound. Turbulence relaxes these constraints, driving structured azimuthal and radial crosstalk dominated by low-order aberration modes.

Thermal engines have been one of the principal topics of thermodynamics ever since its beginning. Today, in the era of the second quantum revolution, although the thermal processes in constant temperatures are relatively well understood in the language of resource theories, a framework to describe thermal engines is still in its infancy. In this work we formalize the resource theory of engines, initially put forward in [Quantum 8, 1222 (2024)], and define such quantities as engine efficiency. Then we turn to a detailed study of thermal engines based on free operations arising from the resource theory of athermality under different restrictions: thermal operation, semilocal thermal operations, and local thermal operation with classical communication. In order to provide analytic lower bounds for thermal engine operation we construct tree states—free states which can be obtained from Gibbs state in simple protocol consisting solely of two-level operations. Furthermore, we derive a full description of the engine based on semilocal thermal operations by describing free states, faithful monotones, and catalytic advantages.

Based on two types of channel divergences, we introduce four kinds of dynamical imaginarity measures, defined respectively by the deviation of a quantum channel from the set of real channels and from its conjugate channel. We show that they satisfy the desirable properties for a valid dynamical imaginarity measure and reduce to corresponding state imaginarity measures for replacement channels. We also compare them with two existing dynamical measures: Choi state-based dynamical imaginarity and the imaginarity-generating power of quantum channels. Furthermore, when specifying these general divergences to the relative entropy, Jensen-Shannon divergence, and trace distance, we establish explicit connections among them and provide an operational interpretation in the context of quantum channel discrimination. Finally, we evaluate the dynamical imaginarity measures induced by conjugate channels for several typical quantum channels.

Higher-dimensional quantum systems, such as qudits, offer architectural and algorithmic advantages over qubits, but their increased spectral crowding and limited controllability render high-fidelity quantum gates particularly challenging. We propose a hybrid optimization framework that integrates optimal control theory methods with contextual deep reinforcement learning to achieve robust controlled-phase gates on two qutrits. Optimal control is first used to design high-fidelity control pulses for a nominal system model. Reinforcement learning is then employed as a calibration stage that learns small residual corrections to these pulses in the presence of static model mismatch, thereby preserving good gate performance under parameter uncertainties and/or variations. By learning structured, low-dimensional residual corrections conditioned on device-specific parameter variations, reinforcement learning enhances the transfer robustness of nominally optimal but parameter-sensitive control solutions across ensembles of devices. Crucially, the reinforcement learning step in our framework does not compete with the optimal control step but provides the adaptability required for realistic hardware, systematically reducing the sensitivity to parameter fluctuations. Our results establish reinforcement learning as a practical and scalable ingredient for robust calibration of quantum gates in high-dimensional systems.

As emerging quantum architectures evolve into heterogeneous networks combining different physical substrates, such as qubits for logic and higher-dimensional qudits for robust communication, the traditional scalar metrics of quantum error correction become insufficient. To address this, we introduce a mathematical framework based on dimension multisets to characterize quantum error-correcting codes and absolutely maximally entangled (AME) states in mixed-dimensional Hilbert spaces. By replacing scalar weights with multisets, we accurately capture the exact physical composition of error supports across these diverse systems. Our central result is the mixed-dimensional quantum MacWilliams identity, which establishes the formal algebraic relationship between Shor-Laflamme enumerators and unitary weight enumerators. From this foundation, we deduce the mixed-dimensional shadow identity and derive rigorous, generalized constraints on code parameters, explicitly formulating the mixed-dimensional quantum Hamming, Singleton, and Scott bounds, and developing a linear program to systematically evaluate code viability. For the Singleton bound, a tighter bound that has no homogeneous analog is derived for pure mixed-dimensional codes. Finally, we deploy this enumerator machinery to thoroughly analyze AME states, utilizing shadow inequalities to constrain their existence and introducing a combinatorial grid method for the explicit construction of mixed-dimensional tripartite AME states.